moved caching from loaders to environment and added environment overlays

--HG--
branch : trunk
diff --git a/jinja2/defaults.py b/jinja2/defaults.py
index 773ad80..848cb8f 100644
--- a/jinja2/defaults.py
+++ b/jinja2/defaults.py
@@ -13,7 +13,20 @@
 from jinja2.utils import generate_lorem_ipsum
 
 
+BLOCK_START_STRING = '{%'
+BLOCK_END_STRING = '%}'
+VARIABLE_START_STRING = '{{'
+VARIABLE_END_STRING = '}}'
+COMMENT_START_STRING = '{#'
+COMMENT_END_STRING = '#}'
+LINE_STATEMENT_PREFIX = None
+
+
 DEFAULT_NAMESPACE = {
     'range':        xrange,
     'lipsum':       generate_lorem_ipsum
 }
+
+
+# export all constants
+__all__ = tuple(x for x in locals() if x.isupper())